Image sensor with an absorption enhancement semiconductor layer

ABSTRACT

An image sensor with an absorption enhancement semiconductor layer is provided. In some embodiments, the image sensor comprises a front-side semiconductor layer, an absorption enhancement semiconductor layer, and a back-side semiconductor layer that are stacked. The absorption enhancement semiconductor layer is stacked between the front-side and back-side semiconductor layers. The absorption enhancement semiconductor layer has an energy bandgap less than that of the front-side semiconductor layer. Further, the image sensor comprises a plurality of protrusions and a photodetector. The protrusions are defined by the back-side semiconductor layer, and the photodetector is defined by the front-side semiconductor layer, the absorption enhancement semiconductor layer, and the back-side semiconductor layer.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/585,044, filed on Nov. 13, 2017, the contents of which areincorporated herein by reference in their entirety.

BACKGROUND

Integrated circuits (ICs) with image sensors are used in a wide range ofmodern day electronic devices, such as, for example, cameras and cellphones. In recent years, complementary metal-oxide semiconductor (CMOS)image sensors have begun to see widespread use, largely replacingcharge-coupled device (CCD) image sensors. Compared to CCD imagesensors, CMOS image sensors are favored due to low power consumption,small size, fast data processing, a direct output of data, and lowmanufacturing cost. Some types of CMOS image sensors include front-sideilluminated (FSI) image sensors and back-side illuminated (BSI) imagesensors.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of some embodiments of animage sensor comprising an absorption enhancement semiconductor layer.

FIGS. 2A-2D illustrate cross-sectional views of various more detailedembodiments of the image sensor of FIG. 1.

FIG. 3 illustrates a top layout view of some embodiments of the imagesensor of FIGS. 1 and 2A-2D.

FIGS. 4A and 4B illustrate cross-sectional views of various embodimentsof the image sensor of FIG. 3.

FIGS. 5-12 illustrate a series of cross-sectional views of someembodiments of a method for forming a front-side illuminated (FSI) imagesensor with an absorption enhancement semiconductor layer.

FIG. 13 illustrates a flowchart of some embodiments of the method ofFIGS. 5-12.

FIGS. 14-21 illustrate a series of cross-sectional views of someembodiments of a method for forming a back-side illuminated (BSI) imagesensor with an absorption enhancement semiconductor layer.

FIG. 22 illustrates a flowchart of some embodiments of the method ofFIGS. 14-21.

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples,for implementing different features of this disclosure. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper”, and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice or apparatus in use or operation in addition to the orientationdepicted in the figures. The device or apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly. Even more, the terms “first”, “second”, “third”, “fourth”,and the like are merely generic identifiers and, as such, may beinterchanged in various embodiments. For example, while an element(e.g., an etch, a dielectric layer, or a substrate) may be referred toas a “first” element in some embodiments, the element may be referred toas a “second” element in other embodiments.

A complementary metal-oxide-semiconductor (CMOS) image sensor maycomprise a semiconductor substrate of monocrystalline silicon and anarray of pixel sensors arranged in the semiconductor substrate. Thepixel sensors comprise respective photodetectors buried in thesemiconductor substrate, and further comprise respective pixeltransistors arranged on a surface of the semiconductor substrate. Thephotodetectors are configured to absorb incident radiation and togenerate an electric signal corresponding to the incident radiation.

A challenge with the CMOS image sensor is that the CMOS image sensorcannot be used outdoors for radiation with a wavelength around 850nanometers (hereafter 850 nanometer radiation). Namely, while the CMOSimage may be used indoors for the 850 nanometer radiation, solarirradiance outdoors introduces noise that decreases the sensitivity ofthe CMOS image sensor to the 850 nanometer radiation. Further, the CMOSimage sensor lacks sufficient sensitivity to the 850 nanometer radiationto compensate for the noise. Another challenge with the CMOS imagesensor is that the monocrystalline silicon has a large energy bandgap,and hence a low absorption coefficient for high wavelength radiation.The high wavelength radiation includes, for example, radiation with awavelength greater than about 900 nanometers. Therefore, the CMOS imagesensor has poor quantum efficiency for high wavelength radiation unlessenhanced. Quantum efficiency (QE) is the fraction of incident photonswhich contribute to the electric signal. For example, the CMOS imagesensor may have a quantum efficiency below 10% for radiation with awavelength around 940 nanometers, and/or may have a quantum efficiencyaround 0% for radiation with a wavelength greater than about 1100nanometers.

One approach for enhancing the CMOS image sensor is to increase athickness of the semiconductor substrate and the depth to which thephotodetectors extend into the semiconductor substrate. The higher thewavelength of incident radiation, the higher the absorption depth.However, this is difficult with existing CMOS processes and adds cost tothe manufacture of the CMOS image sensor. Further, increasing the depthto which the photodetectors extend into the semiconductor substrateincreases cross talk and die size.

In view of the foregoing, various embodiments of the present applicationare directed towards an image sensor with an absorption enhancementsemiconductor layer. In some embodiments, the image sensor comprises afront-side semiconductor layer, an absorption enhancement semiconductorlayer, and a back-side semiconductor layer that are stacked. Theabsorption enhancement semiconductor layer is stacked between thefront-side and back-side semiconductor layers, and has an energy bandgapless than that of the front-side semiconductor layer. In someembodiments, the image sensor further comprises a plurality ofprotrusions and a photodetector. The protrusions are defined by theback-side semiconductor layer. The photodetector is defined by thefront-side and back-side semiconductor layers, as well as the absorptionenhancement semiconductor layer.

Because the absorption enhancement semiconductor layer has a low energybandgap relative to the front-side semiconductor layer, the absorptionenhancement semiconductor layer has a high absorption coefficientrelative to the front-side semiconductor layer. This, in turn, enhancesabsorption of incident radiation by the photodetector, such that thephotodetector has a high QE. The high QE allows the photodetector to beused for high wavelength radiation greater than about 800 nanometers.For example, the photodetector may be used for radiation with awavelength greater than about 900, 940, 1100, or 1375 nanometers. Asanother example, the photodetector may be used outdoors for radiationwith a wavelength between about 840-860 nanometers, notwithstandingnoise from solar irradiance. Further, the high QE allows thephotodetector to be used for the high wavelength radiation without alarge thickness. As such, costs, die size, crosstalk, or any combinationof the foregoing is/are low.

With reference to FIG. 1, a cross-sectional view 100 of some embodimentsof an image sensor comprising an absorption enhanced pixel sensor 102 isprovided. The image sensor may be, for example, front-side illuminated(FSI) or back-side illuminated (BSI). Further, the image sensor may be,for example, a CMOS image sensor, and/or may, for example, be anintegrated circuit (IC) die or chip. The absorption enhanced pixelsensor 102 is configured to absorb incident radiation 104 (e.g., light)with a high wavelength, and is defined by a composite substrate 106 anda passivation layer 108. The high wavelength may, for example, begreater than about 800, 850, 940, 1100, or 1375 nanometers, and/or may,for example, be between about 1.0-2.5 micrometers, between about 0.8-2.5micrometers, or about 0.8-3.0 micrometers.

The passivation layer 108 is on a back-side surface 106 b of thecomposite substrate 106 to protect the back-side surface 106 b. Thepassivation layer 108 may be or comprise, for example, silicon dioxide,silicon nitride, a high κ dielectric, some other dielectric, or anycombination of the foregoing. As used herein, a high κ dielectric maybe, for example, a dielectric with a dielectric constant κ greater thanabout 3.9, 5, 10, or 20. The composite substrate 106 comprises afront-side semiconductor layer 110, an absorption enhancementsemiconductor layer 112, and a back-side semiconductor layer 114.

The front-side semiconductor layer 110 is along and defines a front-sidesurface 106 f of the composite substrate 106 that is opposite theback-side surface 106 b of the composite substrate 106. The back-sidesemiconductor layer 114 is along and defines the back-side surface 106 bof the composite substrate 106. In some embodiments, the back-sidesemiconductor layer 114 directly contacts the passivation layer 108. Thefront-side semiconductor layer 110 and the back-side semiconductor layer114 may be or otherwise comprise, for example, silicon, some othersemiconductor material, or any combination of the foregoing. In someembodiments, the front-side semiconductor layer 110 and/or the back-sidesemiconductor layer 114 is/are devoid of semiconductor atoms other thansilicon atoms.

In some embodiments, the back-side semiconductor layer 114 is orcomprises a semiconductor material with a systematic structure of pores(i.e., a porous semiconductor material). For example, the back-sidesemiconductor layer 114 may be or comprise nanoporous silicon, someother porous silicon, or some other porous semiconductor material. Thepores may be, for example, micrometer sized pores, nanometer sizedpores, smaller sized pores, or any combination of the foregoing. In someembodiments, the pores serve as a light trap for the incident radiation104, thereby enhancing absorption. For example, angled sidewalls of apore may make it difficult for radiation entering the pore tosubsequently reflect out of the pore. Instead, the radiation is morelikely to reflect around inside the pore until being absorbed.

The absorption enhancement semiconductor layer 112 is vertically stackedbetween the front-side semiconductor layer 110 and the back-sidesemiconductor layer 114. In some embodiments, the absorption enhancementsemiconductor layer 112 directly contacts the front-side semiconductorlayer 110 at a front-side heterojunction 116 and/or directly contactsthe back-side semiconductor layer 114 at a back-side heterojunction 118.The absorption enhancement semiconductor layer 112 has a low energybandgap to enhance absorption of the incident radiation 104. In someembodiments, the low energy bandgap is an energy bandgap less than about1.00, 0.80, 0.60, 0.66, or 0.40 electron volt (eV). Further, in someembodiments, the low energy bandgap is an energy bandgap that is lessthan that of the front-side semiconductor layer 110 and/or the back-sidesemiconductor layer 114. For example, the absorption enhancementsemiconductor layer 112 may have an energy bandgap less than about 1.0eV, whereas the front-side semiconductor layer 110 may have an energybandgap greater than about 1.1, 1.5, 2.0, or 5.0 eV. In someembodiments, a difference between an energy bandgap of the absorptionenhancement semiconductor layer 112 and an energy bandgap of thefront-side semiconductor layer 110 is about 0.46 eV or less, is about0.5 eV or less, is between about 0.2-0.5 eV, or is between about 0.4-0.5eV. The absorption enhancement semiconductor layer 112 may, for example,have such a difference with the front-side semiconductor layer 110 whenthe absorption enhancement semiconductor layer 112 is or comprisessilicon germanium and the front-side semiconductor layer 110 is orcomprises silicon. In some embodiments, the absorption enhancementsemiconductor layer 112 also has a direct energy bandgap to furtherenhance absorption of the incident radiation 104. The direct energybandgap may, for example, allow photons 104 p from the incidentradiation 104 to be absorbed without dependence on phonons (e.g.,crystal vibrations). The absorption enhancement semiconductor layer 112may be or comprise, for example, germanium, silicon germanium, silicon(e.g., monocrystalline silicon) doped with a chalcogen, some othersemiconductor material with the low energy bandgap, or any combinationof the foregoing. The chalcogen may be or comprise, for example, sulfur(e.g., S), selenium (e.g., Se), tellurium (e.g., Te), some otherchalcogen, or any combination of the foregoing. In some embodimentswhere the absorption enhancement semiconductor layer 112 is or comprisessilicon doped with the chalcogen, the absorption enhancementsemiconductor layer 112 is devoid of semiconductor atoms other thansilicon atoms. Further, in some embodiments where the absorptionenhancement semiconductor layer 112 is or comprises silicon doped withthe chalcogen, the absorption enhancement semiconductor layer 112 isdoped in excess of the solubility limit of the chalcogen (i.e., issupersaturated) to facilitate sub-band gap absorption of the incidentradiation 104.

In some embodiments, the absorption enhancement semiconductor layer 112has a thickness T of about 10-3000 angstroms, 100-1500 angstroms,1500-3000 angstroms, or a combination of the foregoing. In some of suchembodiments, the absorption enhancement semiconductor layer 112 is orcomprises Si_(0.8)Ge_(0.2). In some of embodiments, the absorptionenhancement semiconductor layer 112 is or comprises Si_(0.8)Ge_(0.2),and/or has a thickness T of about 950-1050 angstroms, about 975-1025angstroms, or about 1000 angstroms. In some embodiments, the front-sidesemiconductor layer 110 has the same crystalline orientation as theback-side semiconductor layer 114.

A photodetector 120 is defined by the composite substrate 106, includingthe absorption enhancement semiconductor layer 112. For example, thephotodetector 120 may comprise a p-type semiconductor region (notshown), an n-type semiconductor region (not shown), and a photo junction(not shown). The p-type semiconductor region and the n-typesemiconductor region are defined by the composite substrate 106, and thephoto junction is defined by the p-type and n-type semiconductorregions. The photo junction may be or comprise, for example, a PNjunction or a PIN junction. The photodetector 120 is configured toconvert the incident radiation 104 to an electric signal. The electricsignal may, for example, result from electron-hole pairs 122 generatedin response to the composite substrate 106 absorbing photons 104 p ofthe incident radiation 104. The photodetector 120 may be, for example, aphotodiode or some other photodetector.

Because the absorption enhancement semiconductor layer 112 has a lowenergy bandgap, the absorption enhancement semiconductor layer 112 has ahigh absorption coefficient. This, in turn, enhances absorption of theincident radiation 104 by photodetector 120, such that the photodetector120 has a high QE. The high QE allows the photodetector 120 to be usedfor high wavelength radiation greater than about 800 nanometers. Forexample, the photodetector 120 may be used for radiation with awavelength greater than about 900, 940, 1100, or 1375 nanometers. Asanother example, the photodetector 120 may be used outdoors forradiation with a wavelength between about 840-860 nanometers,notwithstanding noise from solar irradiance. Further, the high QE allowsthe photodetector 120 to be used for the high wavelength radiationwithout the composite substrate 106 having a large thickness. As such,costs, die size, crosstalk, or any combination of the foregoing is/arelow.

In some embodiments, a plurality of protrusions 124 is defined by thecomposite substrate 106, including the back-side semiconductor layer114. For ease of illustration, only one of the protrusions 124 islabeled 124. The protrusions 124 are along the back-side surface 106 bof the composite substrate 106, and at least partially define anabsorption enhancement structure 126. The protrusions 124 are configuredto prevent incident radiation from reflecting away from thephotodetector 120. For example, the protrusions 124 may have angledsidewalls to prevent such reflectance. By preventing incident radiationfrom reflecting away from the photodetector 120, the protrusions 124increase the amount of radiation absorbed by the photodetector 120,which increases the sensitivity of the photodetector 120.

In some embodiments, the protrusions 124 are in a periodic pattern orarray. In some embodiments, the protrusions 124 each have a cone shape,a pyramid shape, or some other shape. In some embodiments, theprotrusions 124 define a saw-toothed profile. In some embodiments, theprotrusions 124 have a pitch P of about 0.01-8.0 micrometers, about0.2-5.0 micrometers, about 1.0-3.0 micrometers, or any combination ofthe foregoing. In some embodiments, the protrusions 124 have a height Hof about 0.2-20.0 micrometers, about 1.0-15.0 micrometers, about5.0-10.0 micrometers, or any a combination of the foregoing.

In some embodiments where the back-side semiconductor layer 114 is orcomprises the porous semiconductor material, the protrusions 124, andhence the absorption enhancement structure 126, are wholly or partiallydefined by the porous semiconductor material. In some embodiments, thecombination of the porous semiconductor material and the protrusions 124results in the absorption enhancement structure 126 having a directenergy band gap that enhances the quantum efficiency of thephotodetector 120.

With reference to FIG. 2A, a cross-sectional view 200A of some moredetailed embodiments of the image sensor of FIG. 1 is provided. Asillustrated, the front-side semiconductor layer 110 comprises afront-side charge collection region (CCR) 110 c, a floating diffusionregion (FDR) 110 f, and a front-side bulk region 110 b. The front-sidebulk region 110 b surrounds the front-side CCR 110 c and is on oppositesides of the front-side CCR 110 c. Further, the front-side bulk region110 b cups an underside of the FDR 110 f. The FDR 110 f and thefront-side CCR 110 c have a first doping type, and the front-side bulkregion 110 b has a second doping type opposite the first doping type.The first doping type and the second doping type may respectively be,for example, n-type and p-type, or vice versa. The front-sidesemiconductor layer 110 may be or comprise, for example, monocrystallinesilicon, some other crystalline semiconductor material, or some othersemiconductor material.

The absorption enhancement semiconductor layer 112 comprises anabsorption enhancement CCR 112 c and an absorption enhancement bulkregion 112 c, and the back-side semiconductor region 114 comprises aback-side CCR 114 cand a back-side bulk region 114 b. The absorptionenhancement bulk region 112 b surrounds the absorption enhancement CCR112 c and is on opposite sides of the absorption enhancement CCR 112 c.The back-side bulk region 114 b surrounds the back-side CCR 114 cand ison opposite sides of the back-side CCR 114 c. The absorption enhancementCCR 112 c and the back-side CCR 114 chave the first doping type, and theabsorption enhancement bulk region 112 b and the back-side bulk region114 b have the second doping type. The absorption enhancementsemiconductor layer 112 is a semiconductor material with a low energybandgap relative to the front-side semiconductor layer 110 and/or theback-side semiconductor layer 114. The absorption enhancementsemiconductor 114 may be or comprise, for example, germanium, silicongermanium, crystalline silicon doped with a chalcogen, or some othersemiconductor material. The back-side semiconductor region 114 may be orcomprise, for example, silicon or some other semiconductor material. Insome embodiments, the back-side semiconductor region 114 is or comprisesnanoporous silicon or some other porous semiconductor material.

The photodetector 120 is defined by the front-side semiconductor layer110, the absorption enhancement semiconductor layer 112, and theback-side semiconductor layer 114. The photodetector 120 comprises afirst semiconductor region 120 f, a second semiconductor region (notlabeled), and a photo junction 120 p. The first semiconductor region 120f has an opposite doping as the second semiconductor region. The firstsemiconductor region 120 f is defined by the front-side, absorptionenhancement, and back-side CCRs 110 c, 112 c, 114 c . In someembodiments, the first semiconductor region 120 f extends continuouslywith a single doping type from the front-side surface 106 f of thecomposite substrate 106 to the back-side surface 106 b of the compositesubstrate 106. The second semiconductor region is defined by thefront-side, absorption enhancement, and back-side bulk regions 110 b,112 b, 114 b and surrounds the first semiconductor region 120 f. In someembodiments, the second semiconductor region extends continuously with asingle doping type along sidewalls of the first semiconductor region,from the front-side surface 106 f of the composite substrate 106 to theback-side surface 106 b of the composite substrate 106. The photojunction 120 pis defined by the first semiconductor region 120 f and thesecond semiconductor region.

During use of the image sensor, radiation incident on the photodetector120 is absorbed by the first semiconductor region 120 f and the secondsemiconductor region. As discussed above, the absorption enhancementsemiconductor layer 112 and the absorption enhancement structure 126enhance such absorption. In response to absorbing a photon of theradiation, an electron-hole pair is generated. When the electron-holepair is generated within a depletion region of the photo junction 120por a diffusion region of the photo junction 120 p, the electron of thepair and the hole of the pair diffuse and/or drift apart respectively tothe first semiconductor region 120 f and the second semiconductorregion. For example, the electron of the pair diffuses and/or drifts tothe first semiconductor region 120 f, and the hole of the pair diffusesand/or drifts to the second semiconductor region, or vice versa. Suchdrifting may, for example, be caused by an electric field of the photojunction 120 p. Since the second semiconductor region surrounds thefirst semiconductor region 120 f, an electron or hole that diffusesand/or drifts to the first semiconductor region 120 f becomes trapped inthe first semiconductor region 120 f, whereby charge accumulates.

One or more pixel transistors are on the front-side surface 106 f of thecomposite substrate 106 to facilitate readout of the photodetector 120.For example, the one or more pixel transistors may comprise a transfertransistor 202, a source-follower transistor (not shown), a row selecttransistor (not shown), a reset transistor (not shown), some other pixeltransistor, or any combination of the foregoing. The transfer transistor202 is configured to selectively transfer charge accumulated in thefirst semiconductor region 120 f of the photodetector 120 to the FDR 110f. In some embodiments, the transfer transistor 202 comprises a firstsource/drain region (not labeled), a gate electrode 204, a gatedielectric layer 206, and a second source/drain region. The firstsource/drain region may be, for example, wholly or partially defined bythe front-side, absorption enhancement, and back-side CCRs 110 c, 112 c,114 c. The second source/drain region may be wholly or partially definedby, for example, the FDR 110 f.

The gate electrode 204 is between and borders the first and secondsource/drain regions, and is vertically spaced over the compositesubstrate 106 by the gate dielectric layer 206. The gate electrode 204may be or comprise, for example, copper, aluminum copper, some othermetal, doped polysilicon, some other conductive material, or anycombination of the foregoing. The gate dielectric layer 206 may be orcomprise, for example, silicon dioxide, a high κ dielectric, some otherdielectric, or any combination of the foregoing. The first and secondsource/drain regions are doped semiconductor regions of the compositesubstrate 106 having opposite doping types as surrounding regions of thecomposite substrate 106. For example, the first and second source/drainregions may be n-type or p-type.

In some embodiments, an isolation structure 208 extends into thefront-side surface 106 f of the composite substrate 106, and laterallysurrounds the absorption enhanced pixel sensor 102, to electricallyisolate the absorption enhanced pixel sensor 102 from surroundingdevices (not shown). Such surrounding devices may include, for example,other pixel sensors, logic devices, or memory devices. Further, in someembodiments, the isolation structure 208 extends laterally in a closedpath to completely enclose the absorption enhanced pixel sensor 102. Forexample, the isolation structure 208 may have a planar top layout thatis ring shaped. Note that this is not visible within the cross-sectionalview 200A of FIG. 2A. The isolation structure 208 may be or comprise,for example, a shallow trench isolation (STI) structure, a deep trenchisolation (DTI) structure, an implant isolation region, or anycombination of the foregoing.

An interconnect structure 210 overlies the composite substrate 106 andthe one or more pixel transistors (e.g., the transfer transistor 202).Further, where the isolation structure 208 is present, the interconnectstructure 210 overlies the isolation structure 208. The interconnectstructure 210 comprises an interlayer dielectric (ILD) layer 212, aplurality of wires 214, and a plurality of vias 216. For ease ofillustration, only some of the wires 214 are labeled 214 and only someof the vias 216 are labeled 216. The wires 214 and the vias 216 arealternatingly stacked in the ILD layer 212, and define conductive pathsinterconnecting the absorption enhanced pixel sensor 102 and surroundingdevices (not shown). The vias 216 provide vertical routing for theconductive paths, and the wires 214 provide lateral routing for theconductive paths. The wires 214 may be or comprise, for example,aluminum copper, copper, aluminum, some other conductive material, orany combination of the foregoing. The vias 216 may be or comprise, forexample, copper, tungsten, some other conductive material, or anycombination of the foregoing. The ILD layer 212 may or comprise be, forexample, silicon dioxide, silicon nitride, a low κ dielectric (e.g.,fluorosilicate glass (FSG)), some other dielectric, or any combinationof the foregoing. As used herein, a low κ dielectric may be, forexample, a dielectric with a dielectric constant κ less than about 3.9,3.0, 2.0, or 1.0.

With reference to FIG. 2B, a cross-sectional view 200B of some othermore detailed embodiments of the image sensor of FIG. 1 is provided. Asillustrated, FIG. 2B is a variant of FIG. 2A having a front-side bufferlayer 218 and a back-side buffer layer 220.

The front-side buffer layer 218 separates the front-side semiconductorlayer 110 from the absorption enhancement semiconductor layer 112, andmitigates the effects of lattice mismatch between the absorptionenhancement semiconductor layer 112 and the front-side semiconductorlayer 110. Such effects may include, for example, physical strain, whichleads to dark current and white pixels. The front-side buffer layer 218may be or comprise, for example, the same semiconductor material as theabsorption enhancement semiconductor layer 112, the same semiconductormaterial as the front-side semiconductor layer 110, some othersemiconductor material, or any combination of the foregoing. Further,the front-side buffer layer 218 may, for example, comprise an elevatedconcentration of carbon relative to the absorption enhancementsemiconductor layer 112 and/or the front-side semiconductor layer 110.

The back-side buffer layer 220 separates the back-side semiconductorlayer 114 from the absorption enhancement semiconductor layer 112, andmitigates the effects of lattice mismatch between the absorptionenhancement semiconductor layer 112 and the back-side semiconductorlayer 114. The back-side buffer layer 220 may be or comprise, forexample, the same semiconductor material as the absorption enhancementsemiconductor layer 112, the same semiconductor material as back-sidesemiconductor layer 114, some other semiconductor material, or anycombination of the foregoing. Further, the back-side buffer layer 220may, for example, comprise an elevated concentration of carbon relativeto the absorption enhancement semiconductor layer 112 and/or theback-side semiconductor layer 114.

The photodetector 120 is defined by the front-side semiconductor layer110, the absorption enhancement semiconductor layer 112, and theback-side semiconductor layer 114, and is further defined by thefront-side buffer layer 218 and the back-side buffer layer 220. Thefirst semiconductor region 120 f of the photodetector 120 is defined bythe front-side, absorption enhancement, and back-side CCRs 110 c, 112 c,114 c, and is further defined by first regions (not labeled) of thefront-side and back-side buffer layers 218, 220 adjoining thefront-side, absorption enhancement, and back-side CCRs 110 c, 112 c, 114c. The second semiconductor region (not labeled) of the photodetector120 is defined by the front-side, absorption enhancement, and back-sidebulk regions 110 b, 112 b, 114 b, and is further defined by secondregions (not labeled) of the front-side and back-side buffer layers 218,220 adjoining the front-side, absorption enhancement, and back-side bulkregions 110 b, 112 b, 114 b.

With reference to FIG. 2C, a cross-sectional view 200C of some othermore detailed embodiments of the image sensor of FIG. 1 is provided. Asillustrated, FIG. 2C is a variant of FIG. 2A in which the back-sidesemiconductor layer 114 comprises a porous semiconductor layer 222 and acrystalline semiconductor layer 224.

The porous semiconductor layer 222 underlies the crystallinesemiconductor layer 224, and defines the back-side surface 106 b of thecomposite substrate 106. The porous semiconductor layer 222 comprises aporous CCR 222 c and a porous bulk region 222 b. The porous bulk region222 b surrounds the porous CCR 222 c and is on opposite sides of theporous CCR 222 c. Further, the porous bulk region 222 b has an oppositedoping type as the porous CCR 222 c. The porous bulk region 222 b hassame doping type as the front-side and absorption enhancement bulkregions 110 b, 112 b, and the porous CCR 222 c has the same doping typeas the front-side and absorption enhancement CCRs 110 c, 112 c. Theporous semiconductor layer 222 is or comprises nanoporous silicon orsome other semiconductor material with a systematic structure of pores.The pores may be, for example, micrometer sized pores, nanometer sizedpores, smaller sized pores, or any combination of the foregoing.

The crystalline semiconductor layer 224 separates the poroussemiconductor layer 222 from the absorption enhancement semiconductorlayer 112 and is without a systematic structure of the pores. Forexample, the crystalline semiconductor layer 224 may be free of asystematic structure of nanometer and/or micrometer sized pores.Further, the crystalline semiconductor layer 224 comprises a crystallineCCR 224 c and a crystalline bulk region 224 b. The crystalline bulkregion 224 b surrounds the crystalline CCR 224 c and is on oppositesides of the crystalline CCR 224 c. Further, the crystalline bulk region224 bhas an opposite doping type as the crystalline CCR 224 c. Thecrystalline bulk region 224 bhas the same doping type as the front-sideand absorption enhancement bulk regions 110 b, 112 b, and thecrystalline CCR 224 c has the same doping type as the front-side andabsorption enhancement CCRs 110 c, 112 c. The crystalline semiconductorlayer 224 may be, for example, monocrystalline silicon, polycrystallinesilicon, or some other crystalline semiconductor material. Further, thecrystalline semiconductor layer 224 may, for example, have an indirectenergy bandgap.

The protrusions 124, and hence the absorption enhancement structure 126,are at least partially defined by the porous semiconductor layer 222. Insome embodiments, pore size of the porous semiconductor layer 222increases or decreases gradually from tips of the protrusions 124towards the crystalline semiconductor layer 224. For example, pore sizemay decrease gradually from an average size of about 40 nanometers atthe tips to an average size of about 8 nanometers at an interface atwhich the porous semiconductor layer 222 and the crystallinesemiconductor layer 224 directly contact. In some embodiments, theabsorption enhancement structure 126 has a direct energy bandgap due tothe combination of the protrusions 124 and the porous semiconductorlayer 222. This, in turn, allows the absorption enhancement structure126 to absorb incident radiation without phonons (e.g., crystalvibrations).

The photodetector 120 is defined by the front-side semiconductor layer110, the absorption enhancement semiconductor layer 112, the poroussemiconductor layer 222, and the crystalline semiconductor layer 224.The first semiconductor region 120 f of the photodetector 120 is definedby the front-side, absorption enhancement, porous, and crystalline CCRs110 c, 112 c, 222 c, 224 c. The second semiconductor region (notlabeled) of the photodetector 120 is defined by the front-side,absorption enhancement, porous, and crystalline bulk regions 110 b, 112b, 222 b, 224 b. Further, the photodetector 120 overlaps the absorptionenhancement structure 126. Because the photodetector 120 overlaps theabsorption enhancement structure 126, the quantum efficiency of thephotodetector 120 is enhanced by the absorption enhancement structure126. For example, the direct bandgap of the absorption enhancementstructure 126 enhances the quantum efficiency of the photodetector 120.

While FIG. 2C illustrated the back-side semiconductor layer 114 ascomprising both the porous semiconductor layer 222 and the crystallinesemiconductor layer 224, the crystalline semiconductor layer 224 may beomitted in other embodiments. In some of such other embodiments, theporous semiconductor layer 222 may occupy the space presently occupiedby the crystalline semiconductor layer 224 in FIG. 2C.

With reference to FIG. 2D, a cross-sectional view 200D of some othermore detailed embodiments of the image sensor of FIG. 1 is provided. Asillustrated, FIG. 2D is a variant of FIG. 2A in which the back-sidesemiconductor layer 114 comprises the porous semiconductor layer 222 andthe crystalline semiconductor layer 224, and in which the front-sidebuffer layer 218 and the back-side buffer layer 220 separate theabsorption enhancement semiconductor layer 112 respectively from thefront-side semiconductor layer 110 and the back-side semiconductor layer114. The porous semiconductor layer 222 and the crystallinesemiconductor layer 224 may, for example, be as described with regard toFIG. 2C, and the front-side buffer layer 218 and the back-side bufferlayer 220 may, for example, be as described with regard to FIG. 2B.

With reference to FIG. 3, a top layout view 300 of some embodiments ofthe image sensor of FIGS. 1 and 2A-2D is provided. As illustrated, alogic region 302 laterally surrounds a pixel sensor array 304. The logicregion 302 comprises, for example, logic and/or memory devices (notshown) configured to read and/or store data generated by the pixelsensor array 304 in response to incident radiation.

The pixel sensor array 304 is made up of a plurality of absorptionenhanced pixel sensors 306 arranged in X rows and Y columns. For ease ofillustration, only some of the absorption enhanced pixel sensors 306 arelabeled 306. X and Y are integer values greater than zero and may be,for example, the same or different. For example, X and Y may both be128, 256, 512, 1024, 2048, or 4096. As another example, X and Y mayrespectively be 768 and 1024, 1024 and 2048, 256 and 1024, 512 and 128,or 4096 and 1024. The absorption enhanced pixel sensors 306 are eachindividually configured as the absorption enhanced pixel sensor 102 isconfigured in any one of the FIGS. 1 and 2A-2D.

An isolation structure 208 laterally surrounds the pixel sensor array304 to electrically isolate the pixel sensor array 304 from the logicregion 302. Further, the isolation structure 208 laterally surroundseach of the absorption enhanced pixel sensors 306 to electricallyisolate the absorption enhanced pixel sensors 306 from each other.

With reference to FIGS. 4A and 4B, cross-sectional views 400A, 400B ofvarious embodiments of the image sensor of FIG. 3 are provided. Thecross-sectional views 400A, 400B may, for example, be taken along line Ain FIG. 3.

As illustrated by the cross-sectional view 400A of FIG. 4A, the imagesensor is FSI, and comprises a plurality of absorption enhanced pixelsensors 306 a-306 c. The absorption enhanced pixel sensors 306 a-306 care individually configured according the absorption enhanced pixelsensor 102 of FIG. 2A, but may alternatively be individually configuredaccording to the absorption enhanced pixel sensor 102 of any one ofFIGS. 1 and 2B-2D. Further, the absorption enhanced pixel sensors 306a-306 c are covered by the interconnect structure 210, and are eachconfigured to receive incident radiation 104 through the front-sidesurface 106 f and the interconnect structure 210. In some embodiments,the wires 214 of the interconnect structure 210 and the vias 216 of theinterconnect structure 210 are limited to sides of the photodetectors120 of the absorption enhanced pixel sensors 306 a-306 c (i.e., are notdirectly over the photodetectors 120) so radiation is not blocked fromimpinging on the photodetectors 120. For ease of illustrated, only someof the wires 214 are labeled 214, only some of the vias 216 are labeled216, and only some of the photodetectors 120 are labeled 120.

In some embodiments, a carrier substrate 402 underlies and is bonded tothe passivation layer 108. The carrier substrate 402 may be, forexample, a bulk silicon substrate, some other bulk semiconductorsubstrate, or some other substrate. In some embodiments, color filters404 respectively cover the absorption enhanced pixel sensors 306 a-306c. The color filters 404 are configured to pass assigned wavelengths ofradiation, while blocking unassigned wavelengths of radiation. Forexample, a color filter may be configured to pass red wavelengths ofradiation, while blocking blue wavelengths of radiation, whereas anothercolor filter may be configured to pass blue wavelengths of radiation,while blocking red wavelengths of radiation. In some embodiments, thecolor filters 404 are respectively covered by micro-lenses 406configured to focus incident radiation on the photodetectors 120 of theabsorption enhanced pixel sensors 306 a-306 c, respectively. For ease ofillustration, only some of the color filters 404 are labeled 404, andonly some of the micro-lenses 406 are labeled 406.

As illustrated by the cross-sectional view 400B of FIG. 4B, a variant ofFIG. 4A is provided in which the image sensor is BSI. In contrast withFIG. 4A, the absorption enhanced pixel sensors 306 a-306 c overlie theinterconnect structure 210. Further, the absorption enhanced pixelsensors 306 a-306 c are each configured to receive incident radiation104 through the back-side surface 106 b and the passivation layer 108.As with FIG. 2A, the absorption enhanced pixel sensors 306 a-306 c areindividually configured according the absorption enhanced pixel sensor102 of FIG. 2A, but may alternatively be individually configuredaccording to the absorption enhanced pixel sensor 102 of any one ofFIGS. 1 and 2B-2D.

With reference to FIGS. 5-12, a series of cross-sectional views 500-1200of some embodiments of a method for forming a FSI image sensor withabsorption enhanced pixel sensors is provided. For example, the methodmay be employed to form the image sensor of FIG. 4A. As noted above, theimage sensor of FIG. 4A is illustrated with regard to the embodiments ofFIG. 2A, but may alternatively be employed with the embodiments of anyone of FIGS. 1 and 2B-2D. Therefore, it is to be understood that themethod may also be employed with the embodiments of any one of FIGS. 1and 2B-2D.

As illustrated by the cross-sectional view 500 of FIG. 5, a front-sidesemiconductor layer 110 is provided. In some embodiments, the front-sidesemiconductor layer 110 is a bulk silicon substrate, a bulksemiconductor substrate, or some other semiconductor substrate. In someembodiments, the front-side semiconductor layer 110 is or comprisesmonocrystalline silicon, some other silicon, or some other semiconductormaterial. In some embodiments, the front-side semiconductor layer 110has a single material. In some embodiments, the front-side semiconductorlayer 110 has an n-type or p-type doping. In some embodiments, thefront-side semiconductor layer 110 has a single doping type.

Also illustrated by the cross-sectional view 500 of FIG. 5, anabsorption enhancement semiconductor layer 112 is formed covering afront-side semiconductor layer 110. The absorption enhancementsemiconductor layer 112 has a low energy bandgap. In some embodiments,the low energy bandgap is an energy bandgap that is less than about 1.00eV, 0.80 eV, 0.60 eV, 0.66 eV, or 0.40 eV. Further, in some embodiments,the low energy bandgap is an energy bandgap that is less than that ofthe front-side semiconductor layer 110. In some embodiments, theabsorption enhancement semiconductor layer 112 also has a direct energybandgap. As a result of the low energy bandgap and/or the direct energybandgap, the absorption enhancement semiconductor layer 112 has a highabsorption coefficient, which enhances the QE of photodetectorshereafter formed. In some embodiments, the absorption enhancementsemiconductor layer 112 has an n-type or p-type doping. In someembodiments, the absorption enhancement semiconductor layer 112 has thesame doping type as the front-side semiconductor layer 110, and/or has asingle doping type. The absorption enhancement semiconductor layer 112may be or comprise, for example, germanium, silicon germanium,crystalline silicon doped with a chalcogen, some other semiconductormaterial with the low energy bandgap, or any combination of theforegoing. The chalcogen may be or comprise, for example, sulfur (e.g.,S), selenium (e.g., Se), tellurium (e.g., Te), some other chalcogen, orany combination of the foregoing.

In some embodiments, the absorption enhancement semiconductor layer 112is formed by doping a bulk semiconductor substrate with the chalcogen.In such embodiments, the absorption enhancement semiconductor layer 112is a chalcogen doped region of the bulk semiconductor substrate, and thefront-side semiconductor layer 110 is a remainder of the bulksemiconductor substrate. The doping may, for example, be performed byion implantation or some other doping process. Further, in someembodiments where the absorption enhancement semiconductor layer 112 isdoped with the chalcogen, the absorption enhancement semiconductor layer112 is doped in excess of the solubility limit (i.e., is supersaturated)to facilitate sub-band gap absorption of photons. The supersaturationmay, for example, be performed by ion implantation followed by pulsedlaser melting (e.g., about 1-7, 2-6, 3-5, or 4 laser shots).

In other embodiments, the absorption enhancement semiconductor layer 112is formed by epitaxy. The epitaxy may, for example, be performed bychemical vapor deposition (CVD), physical vapor deposition (PVD), someother deposition process, molecular beam epitaxy, or some otherepitaxial process. In some of the epitaxial embodiments, the absorptionenhancement semiconductor layer 112 is or comprises silicon germanium orgermanium. Alternatingly, in some of the epitaxial embodiments, theabsorption enhancement semiconductor layer 112 is or comprisescrystalline silicon doped with the chalcogen. The doping may, forexample, be performed by ion implantation or some other doping process.Further, in some of the epitaxial embodiments, the absorptionenhancement semiconductor layer 112 is supersaturated with the chalcogenas described above.

Also illustrated by the cross-sectional view 500 of FIG. 5, a back-sidesemiconductor layer 114 is formed covering the absorption enhancementsemiconductor layer 112. In some embodiments, the back-sidesemiconductor layer 114 has an n-type or p-type doping. In someembodiments, the back-side semiconductor layer 114 has a single dopingtype. In some embodiments, the back-side semiconductor layer 114 has thesame doping type as the front-side semiconductor layer 110 and/or theabsorption enhancement semiconductor layer 112. In some embodiments, theback-side semiconductor layer 114 has a higher energy bandgap than theabsorption enhancement semiconductor layer 112. The back-sidesemiconductor layer 114 may be, for example, monocrystalline silicon,porous silicon, some other semiconductor material, the samesemiconductor material as the front-side semiconductor layer 110, or anycombination of the foregoing. In some embodiments, the back-sidesemiconductor layer 114 is a single material. The back-sidesemiconductor layer 114 may, for example, be formed by CVD, PVD, someother deposition process, molecular beam epitaxy, or some otherepitaxial process.

In some embodiments, a thickness T of the absorption enhancementsemiconductor layer 112 is controlled so the back-side semiconductorlayer 114 has the same crystalline direction as the front-sidesemiconductor layer 110. For example, if the thickness T of theabsorption enhancement semiconductor layer 112 is large, the back-sidesemiconductor layer 114 may have a different crystalline direction thanthe front-side semiconductor layer 110. Additionally, in someembodiments in which the absorption enhancement semiconductor layer 112is or comprises silicon germanium, a concentration of germanium in theabsorption enhancement semiconductor layer 112 is controlled so theback-side semiconductor layer 114 has the same crystalline direction asthe front-side semiconductor layer 110. In some embodiments, theabsorption enhancement semiconductor layer 112 is or comprisesSi_(0.8)Ge_(0.2) and has a thickness T between about 1000-3000angstroms, about 900-1100 angstroms, or about 500-1500 angstroms so theback-side semiconductor layer 114 has the same crystalline direction asthe front-side semiconductor layer 110.

The back-side semiconductor layer 114, the absorption enhancementsemiconductor layer 112, and the front-side semiconductor layer 110define a composite substrate 106. The front-side semiconductor layer 110is along and defines a front-side surface 106 f of the compositesubstrate 106, and the back-side semiconductor layer 114 is along anddefines a back-side surface 106 b of the composite substrate 106. Theback-side surface 106 b and the front-side surface 106 f are on oppositesides of the composite substrate 106.

As illustrated by the cross-sectional view 600 of FIG. 6, a plurality ofprotrusions 124 is formed in the composite substrate 106, including theback-side semiconductor layer 114. For ease of illustration, only someof the protrusions 124 are labeled 124. The protrusions 124 at leastpartially define an absorption enhancement structure 126, and preventincident radiation from reflecting away the composite substrate 106. Forexample, the protrusions 124 may have angled sidewalls to prevent suchreflectance. As a result, the protrusions 124 enhance the sensitivity ofphotodetectors hereafter formed in composite substrate 106. In someembodiments, the protrusions 124 are in a periodic pattern or array. Insome embodiments, the protrusions 124 each have a cone shape, a pyramidshape, or some other shape. In some embodiments, the protrusions 124define a saw-toothed profile.

In some embodiments, a process for forming the protrusions 124 comprisesdry etching the back-side semiconductor layer 114. For example, aphotoresist layer 602 may be formed on the back-side semiconductor layer114, and patterned with a pattern of the protrusions 124. A dry etchantmay then be applied to the back-side semiconductor layer 114 with thephotoresist layer 602 in place to form the protrusions 124, and thephotoresist layer 602 may be subsequently stripped. In some embodiments,the dry etchant damages the back-side semiconductor layer 114, therebyresulting in electron traps along the back-side surface 106 b of acomposite substrate 106. The electron traps may, in turn, lead toleakage current, dark current, and white pixels. Therefore, in someembodiments, the process for forming the protrusions 124 furthercomprises wet etching the back-side semiconductor layer 114 to removethe damage. For example, a wet etchant may be applied to the back-sidesemiconductor layer 114 before or after the photoresist layer 602 isstripped. The wet etchant may be, for example, tetramethylammoniumhydroxide (TMAH) or some other wet etchant.

In some embodiments, a systematic structure of pores is formed in theback-side semiconductor layer 114. The pores may be, for example,micrometer sized pores, nanometer sized pores, smaller sized pores, orany combination of the foregoing. The pores serve as a light trap forthe incident radiation, thereby enhancing QE of photodetectors hereafterformed. Further, in some embodiments, the combination of the systematicstructure of pores and the protrusions 124 results in the absorptionenhancement structure 126 having a direct energy bandgap, which alsoenhances QE of the photodetectors

In some embodiments, a process for forming the systematic structure ofpores comprises performing an etch into the back-side semiconductorlayer 114. The etch may, for example, be performed by applying asolution of hydrofluoric acid to the back-side semiconductor layer 114,and subsequently activating the hydrofluoric acid. The hydrofluoric acidmay, for example, be activated: electrochemically by applying asufficient voltage to the solution (anodic etching); electrochemicallyby adding an oxidant with a sufficient standard electrode potential tothe solution (stain etching); or photoelectrochemically by irradiatingthe solution with a laser or lamp with a sufficiently short wavelengthto excite electrons in the solution to the conduction band. In someembodiments, the solution further includes a ferric nitride. The processfor forming the systematic structure of pores may be formed before,during, or after the process for forming the protrusions 124.

As illustrated by cross-sectional view 700 of FIG. 7, a passivationlayer 108 is formed covering the back-side surface 106 b of thecomposite substrate 106. The passivation layer 108 has an inner surface108 i facing the back-side surface 106 b of the composite substrate 106,and further has an outer surface 108 o opposite the inner surface 108 i.In some embodiments, the inner surface 108 i of the passivation layer108 is conforms and/or directly contacts the back-side surface 106 b ofthe composite substrate 106. In some embodiments, the outer surface 108o of the passivation layer 108 is substantially flat and/orsubstantially planar. The passivation layer 108 may be or comprise, forexample, silicon dioxide, silicon nitride, a high κ dielectric, someother dielectric, or any combination of the foregoing.

In some embodiments, a process for forming the passivation layer 108comprises depositing the passivation layer 108 on the back-side surface106 b of the composite substrate 106, and subsequently performing aplanarization into the outer surface 108 o of the passivation layer 108.The depositing may, for example, be performed by thermal oxidation, CVD,PVD, sputtering, or some other deposition process. The planarizationmay, for example, be performed by a chemical mechanical polish (CMP) orsome other planarization process.

As illustrated by the cross-sectional view 800 of FIG. 8, in someembodiments, the back-side surface 106 b of the composite substrate 106is bonded to a carrier substrate 402 through the passivation layer 108.The carrier substrate 402 may be, for example, a bulk monocrystallinesilicon substrate, some other silicon substrate, a SOI substrate, someother semiconductor substrate, or some other substrate. In someembodiments, the bonding process is performed by a fusion bondingprocess or some other bonding process.

As illustrated by the cross-sectional view 900 of FIG. 9, the structureof FIG. 8 is flipped vertically. Further, the composite substrate 106 isthinned at the front-side surface 106 f of the composite substrate 106,thereby reducing a thickness of the composite substrate 106 and athickness of the front-side semiconductor layer 110. The thinningreduces a thickness of the composite substrate 106 from a firstthickness T₁ to a second thickness T₂. The second thickness T₂ may, forexample, be less than a third thickness T₃ of the carrier substrate 402.In some embodiments, the composite substrate 106 is thinned byperforming an etch and/or a planarization into the front-sidesemiconductor layer 110. The planarization may, for example, beperformed by a CMP or some other planarization process.

As illustrated by the cross-sectional view 1000 of FIG. 10, a pluralityof absorption enhanced pixel sensors 306 a-306 c are formed on thefront-side surface 106 f of the composite substrate 106. Each of theabsorption enhanced pixel sensors 306 a-306 c comprises a photodetector120 defined by the front-side semiconductor layer 110, the absorptionenhancement semiconductor layer 112, and the back-side semiconductorlayer 114. For ease of illustration, only some of the photodetectors 120are labeled 120. Each photodetector 120 of the absorption enhanced pixelsensors 306 a-306 c comprises a first semiconductor region 120 f, asecond semiconductor region (not labeled), and a photo junction 120 p.For ease of illustration, only some of the first semiconductor regions120 f are labeled 120 f, and only some of the photo junctions 120 parelabeled 120 p. The first semiconductor region 120 f has a first dopingtype, and the second semiconductor region has a second doping typeopposite the first doping type. The first semiconductor region 120 f issurrounded by the second semiconductor region, and the firstsemiconductor region 120 f and the second semiconductor region partiallyor wholly define the photo junction 120 p. The photo junction 120 pmaybe, for example, a PN junction, a PIN junction, or some other photojunction.

In some embodiments, each first semiconductor region 120 f of theabsorption enhanced pixel sensors 306 a-306 c is defined wholly orpartially by a front-side CCR 110 c of the front-side semiconductorlayer 110, an absorption enhancement CCR 112 c of the absorptionenhancement semiconductor layer 112, and a back-side CCR 114 cof theback-side semiconductor layer 114. For ease of illustration, only someof the front-side CCRs 110 c are labeled 110 c, only some of theabsorption enhancement CCRs 112 c are labeled 112 c, and only some ofthe back-side CCRs 114 care labeled 114 c. In some embodiments, thesecond semiconductor regions of the of the absorption enhanced pixelsensors 306 a-306 c are defined wholly or partially by a front-side bulkregion 110 b of the front-side semiconductor layer 110, an absorptionenhancement bulk region 112 b of the absorption enhancementsemiconductor layer 112, and a back-side bulk region 114 b of theback-side semiconductor layer 114.

In some embodiments, a process for forming the photodetectors 120comprises implanting dopants into the composite substrate 106 to formthe first semiconductor regions 120 f. In some of such embodiments, thecomposite substrate 106 initially has a single doping type, such thatthe doping is performed using dopants of an opposite doping type. Thedoping may, for example, be performed using ion implantation or someother doping process, and/or may, for example, use a photoresist mask toselectively implant the dopants.

Each of the absorption enhanced pixel sensors 306 a-306 c furthercomprises a transfer transistor 202. For ease of illustration, only someof the transfer transistors 202 are labeled 202. Each transfertransistor 202 of the absorption enhanced pixel sensors 306 a-306 ccomprises a gate electrode 204, a gate dielectric layer 206, a firstsource/drain region (not labeled), and a second source/drain region (notlabeled). For ease of illustration, only some of the gate electrodes 204are labeled 204, and only some of the gate dielectric layers 206 arelabeled 206. The gate dielectric layer 206 overlies the front-sidesemiconductor layer 110, and the gate electrode 204 overlies the gatedielectric layer 206. The first and second source/drain regions are atleast partially defined by the front-side semiconductor layer 110 andhave the same doping type, which is opposite that of the front-side bulkregion 110 b. Further, the first and second source/drain regions arerespectively on opposite sides of the gate electrode 204, such that thegate electrode 204 is sandwiched between the first and secondsource/drain regions. The first source/drain region may be defined by,for example, a respective one of the first semiconductor regions 120 fof the photodetectors 120. The second source/drain region may be definedby, for example, a floating diffusion region (FDR) 110 f of thefront-side semiconductor layer 110.

In some embodiments, a process for forming the transfer transistors 202is performed before forming the photodetectors 120. In otherembodiments, the process for forming the transfer transistors 202 isperformed while forming the photodetectors 120. Further, in someembodiments, the process for forming the transfer transistors 202comprises forming a dielectric layer and an electrode layer stacked onthe front-side semiconductor layer 110. The dielectric layer may beformed by, for example, thermal oxidation, PVD, CVD, sputtering, someother deposition process, or any combination of the foregoing. Theelectrode layer may be formed by, for example, electrochemical plating,electroless plating, PVD, CVD, sputtering, some other deposition orplating process, or any combination of the foregoing. Further, in someembodiments, the process for forming the transfer transistors 202comprises performing an etch into the dielectric layer and the electrodelayer to define the gate electrodes 204 and the gate dielectric layers206, and subsequently implanting dopants into the front-sidesemiconductor layer 110 to define the first and second source/drainregions.

Also illustrated by the cross-sectional view 1000 of FIG. 10, anisolation structure 208 is formed extending into the front-sidesemiconductor layer 110 to separate and surround the absorption enhancedpixel sensors 306 a-306 c. In some embodiments, the isolation structure208 is formed prior to forming the absorption enhanced pixel sensors 306a-306 c. The isolation structure 208 may, for example, be formed as anSTI structure, a DTI structure, an implant isolation region, or someother isolation structure. In some embodiments, the process for formingthe isolation structure 208 comprises etching the front-sidesemiconductor layer 110 to form trenches, and subsequently filling thetrenches with a dielectric material.

As illustrated by the cross-sectional view 1100 of FIG. 11, aninterconnect structure 210 is formed covering the the absorptionenhanced pixel sensors 306 a-306 c and the composite substrate 106. Theinterconnect structure 210 comprises an ILD layer 212, a plurality ofwires 214, and a plurality of vias 216. For ease of illustration, onlysome of the wires 214 are labeled 214, and only some of the vias 216 arelabeled 216. The wires 214 are alternatingly stacked with the vias 216in the ILD layer 212 to define conductive paths electrically couplingthe absorption enhanced pixel sensors 306 a-306 c to each other and/orother devices.

In some embodiments, a process for forming the interconnect structure210 comprises repeatedly forming an ILD sublayer (i.e., a sublayer ofthe ILD layer 212) over the composite substrate 106, performing aplanarization into a top surface of the ILD sublayer, selectivelyetching the ILD sublayer to form a via opening and/or a wire opening,and filling the via opening and/or the wire opening with a conductivematerial. The ILD sublayer may, for example, be formed by thermaloxidation, CVD, PVD, sputtering, some other deposition process, or anycombination of the foregoing. The planarization may, for example, beperformed by a CMP or some other planarization process. The selectiveetching may, for example, be performed “selectively” usingphotolithography. The filling may, for example, be performed by CVD,PVD, electroplating, electro-less plating, some other deposition orplating process, or any combination of the foregoing. In someembodiments, the process for forming the interconnect structure 210comprises repeatedly performing a dual-damascene process or asingle-damascene process to form the wires 214 and the vias 216. Thedual-damascene process forms two conductive features (e.g., a wire and avia) at the same time, whereas the single-damascene process forms asingle conductive feature (e.g., a wire or a via) at a time.

As illustrated by the cross-sectional view 1200 of FIG. 12, a pluralityof color filters 404 is formed on the interconnect structure 210. Forease of illustration, only some of the color filters 404 are labeled404. The color filters 404 respectively overlie the absorption enhancedpixel sensors 306 a-306 c, and each passes assigned wavelengths ofradiation while blocking unassigned wavelengths of radiation. In someembodiments, the color filters 404 are formed by forming a color filterlayer for first wavelengths of radiation, patterning the color filterlayer, and then repeating the foregoing for different wavelengths ofradiation.

Also illustrated by the cross-sectional view 1200 of FIG. 12, aplurality of micro-lenses 406 is formed on the color filters 404. Forease of illustration, only some of the micro-lenses 406 are labeled 406.The micro-lenses 406 respectively overlie the color filters 404, andfocus incident radiation respectively on the absorption enhanced pixelsensors 306 a-306 c. In some embodiments, a process for forming themicro-lenses 406 comprises forming a micro-lens layer over the colorfilters 404, and subsequently patterning the micro-lens layer intomicro-lens templates individual to the micro-lenses 406.

While not illustrated, an additional absorption enhancementsemiconductor layer may be formed conforming to the protrusions 124,between the passivation layer 108 and the back-side semiconductor layer,in other embodiments. The additional absorption enhancementsemiconductor layer may, for example, be as the absorption enhancementsemiconductor layer 112 is described with regard to FIG. 5, and/or may,for example, be formed as the absorption enhancement semiconductor layer112 is described with regard to FIG. 5. A challenge with these otherembodiments is that other devices on the composite substrate 106 (see,e.g., the logic region 302 of FIG. 3) may limit temperatures used whileforming the additional absorption enhancement semiconductor layer.Exceeding the temperature limit may damage the other devices. As aresult of the temperature limit, the additional absorption enhancementsemiconductor layer may have a poor crystalline quality and/or dopantsthat are not fully activated, which may lead to leakage current, darkcurrent, and white pixels. Note that since the absorption enhancementsemiconductor layer 112 is formed early in the method, the absorptionenhancement semiconductor layer 112 is not subject to the temperaturelimit.

With reference to FIG. 13, a flowchart 1300 of some embodiments of themethod of FIGS. 5-12 is provided.

At 1302, an absorption enhancement semiconductor layer is formed on afront-side semiconductor layer. The absorption enhancement semiconductorlayer has an energy bandgap less than that of the front-sidesemiconductor layer. See, for example, FIG. 5. The high absorption layermay be, for example, germanium, silicon germanium, or monocrystallinesilicon dope with a chalcogen. Further, the high absorption layer may,for example, have a direct energy bandgap or an indirect energy bandgap.

Also at 1302, a back-side semiconductor layer is formed on theabsorption enhancement semiconductor layer. The front-side semiconductorlayer, the absorption enhancement semiconductor layer, and the back-sidesemiconductor layer collectively define a composite substrate. See, forexample, FIG. 5.

At 1304, a plurality of protrusions is formed in the back-sidesemiconductor layer, along a back-side surface of the compositesubstrate. The protrusions at least partially define an absorptionenhancement structure. See, for example, FIG. 6.

At 1306, a passivation layer is formed on the back-side surface of thecomposite substrate. See, for example, FIG. 7.

At 1308, a carrier substrate is bonded to the back-side surface of thecomposite substrate through the passivation layer. See, for example,FIG. 8.

At 1310, the composite substrate is thinned at a front-side surface ofthe composite substrate, thereby reducing a thickness of the compositesubstrate and a thickness of the front-side semiconductor layer. See,for example, FIG. 9.

At 1312, a pixel sensor is formed on the front-side surface of thecomposite substrate. The pixel sensor comprises a photodetectorpartially defined by the absorption enhancement semiconductor layer andthe absorption enhancement structure. See, for example, FIG. 10. Becausethe absorption enhancement semiconductor layer has the low energybandgap, the absorption enhancement semiconductor has a high absorptioncoefficient. This, in turn, enhances absorption of incident radiation byphotodetector, such that the photodetector has a high QE. The high QEallows the photodetector to be used for high wavelength radiationgreater than about 800 nanometers. Further, the high QE allows thephotodetector to be used for the high wavelength radiation without alarge thickness. As such, costs, die size, crosstalk, or any combinationof the foregoing is/are low.

At 1314, an interconnect structure is formed covering the pixel sensoron the front-side surface of the composite substrate. The interconnectstructure comprises an ILD layer, wires, and vias, and the wires and thevias are alternatingly stacked in the ILD layer. See, for example, FIG.11.

At 1316, a color filter and a micro-lens are formed stacked on theinterconnect structure. See, for example, FIG. 12.

While the flowchart 1300 of FIG. 13 is illustrated and described hereinas a series of acts or events, it will be appreciated that theillustrated ordering of such acts or events is not to be interpreted ina limiting sense. For example, some acts may occur in different ordersand/or concurrently with other acts or events apart from thoseillustrated and/or described herein. Further, not all illustrated actsmay be required to implement one or more aspects or embodiments of thedescription herein, and one or more of the acts depicted herein may becarried out in one or more separate acts and/or phases.

With reference to FIGS. 14-21, a series of cross-sectional views1400-2100 of some embodiments of a method for forming a BSI image sensorwith absorption enhanced pixel sensors is provided. For example, themethod may be employed to form the image sensor of FIG. 4B. As notedabove, the image sensor of FIG. 4B is illustrated with regard to theembodiments of FIG. 2A, but may alternatively be employed with theembodiments of any one of FIGS. 1 and 2B-2D. Therefore, it is to beunderstood that the method may also be employed with the embodiments ofany one of FIGS. 1 and 2B-2D.

As illustrated by the cross-sectional view 1400 of FIG. 14, a back-sidesemiconductor layer 114 is provided or formed. In some embodiments, theback-side semiconductor layer 114 is or comprises a bulk siliconsubstrate, some other bulk semiconductor substrate, or some othersemiconductor substrate. The back-side semiconductor layer 114 may be orcomprise, for example, monocrystalline silicon, nanoporous silicon, someother silicon, some other semiconductor material, or any combination ofthe foregoing. Further, the back-side semiconductor layer 114 may, forexample, have an n-type or p-type doping.

In some embodiments, the back-side semiconductor layer 114 comprises afirst back-side semiconductor sublayer 114 f and a second back-sidesemiconductor sublayer 114 s covering the first back-side semiconductorsublayer 114 f. The first back-side semiconductor sublayer 114 f may beor comprise, for example, a bulk silicon substrate, a bulk semiconductorsubstrate, or some other substrate. The first back-side semiconductorsublayer 114 f and the second back-side semiconductor sublayer 114 s maybe or comprise, for example, monocrystalline silicon, nanoporoussilicon, some other semiconductor material, or any combination of theforegoing. Further, the first back-side semiconductor sublayer 114 f andthe second back-side semiconductor sublayer 114 s may, for example, havean n-type or p-type doping. In some embodiments, the first back-sidesemiconductor sublayer 114 f and the second back-side semiconductorsublayer 114 s are the same semiconductor material and have the samedoping type. Further, in some of such embodiments, the first back-sidesemiconductor sublayer 114 f has a greater doping concentration than thesecond back-side semiconductor sublayer 114 s, or vice versa

In some embodiments, the back-side semiconductor layer 114 is formed byproviding the first back-side semiconductor sublayer 114 f, andsubsequently depositing the second back-side semiconductor sublayer 114s on the first back-side semiconductor sublayer 114 f. The depositingmay, for example, be performed by CVD, PVD, molecular beam epitaxy, orsome other deposition process. In other embodiments, the back-sidesemiconductor sublayer 114 is formed by providing a bulk semiconductorsubstrate, and subsequently doping the bulk semiconductor substrate. Insuch embodiments, the second back-side semiconductor sublayer 114 s isthe doped portion of the bulk semiconductor substrate, and the firstback-side semiconductor sublayer 114 f is a remainder of the bulksemiconductor substrate. The doping may, for example, be performed byion implantation of some other doping process.

Also illustrated by the cross-sectional view 1400 of FIG. 14, anabsorption enhancement semiconductor layer 112 is formed covering theback-side semiconductor layer 110. The absorption enhancementsemiconductor layer 112 has a low energy bandgap that enhances quantumefficiency of photodetectors hereafter formed. In some embodiments, thelow energy bandgap is an energy bandgap that is less than about 1.00 eV,0.80 eV, 0.60 eV, 0.66 eV, or 0.40 eV. Further, in some embodiments, thelow energy bandgap is an energy bandgap that is less than that of theback-side semiconductor layer 114. In some embodiments, the absorptionenhancement semiconductor layer 112 also has a direct energy bandgap.The absorption enhancement semiconductor layer 112 may be or comprise,for example, germanium, silicon germanium, crystalline silicon dopedwith a chalcogen, or some other semiconductor material with the lowenergy bandgap. Further, the absorption enhancement semiconductor layer112 may, for example, be as described with regard to FIG. 5.

In some embodiments, the absorption enhancement semiconductor layer 112is formed by epitaxy. The epitaxy may, for example, be performed by CVD,PVD, some other deposition process, molecular beam epitaxy, or someother epitaxial process. In some embodiments, the absorption enhancementsemiconductor layer 112 is doped with the chalcogen by ion implantationor some other doping process. Further, in some embodiments, theabsorption enhancement semiconductor layer 112 is supersaturated withthe chalcogen. The supersaturation may, for example, be performed by ionimplantation, followed by pulsed laser melting (e.g., about 1-7, 2-6,3-5, or 4 laser shots).

Also illustrated by the cross-sectional view 1400 of FIG. 14, afront-side semiconductor layer 110 is formed covering the absorptionenhancement semiconductor layer 112. The front-side semiconductor layer110 may be or comprise, for example, monocrystalline silicon, some othersemiconductor material, or any combination of the foregoing. Further,the front-side semiconductor layer 110 may, for example, be formed byCVD, PVD, some other deposition process, molecular beam epitaxy, or someother epitaxial process.

In some embodiments, a thickness T of the absorption enhancementsemiconductor layer 112 is controlled so the front-side semiconductorlayer 110 has the same crystalline direction as the back-sidesemiconductor layer 114. For example, if the thickness T of theabsorption enhancement semiconductor layer 112 is large, the front-sidesemiconductor layer 110 may have a different crystalline direction thanthe back-side semiconductor layer 114. Additionally, in some embodimentsin which the absorption enhancement semiconductor layer 112 is orcomprises silicon germanium, a concentration of germanium in theabsorption enhancement semiconductor layer 112 is controlled so thefront-side semiconductor layer 110 has the same crystalline direction asthe back-side semiconductor layer 114. In some embodiments, theabsorption enhancement semiconductor layer 112 is or comprisesSi_(0.8)Ge_(0.2) and has a thickness T between about 1000-3000angstroms, about 900-1100 angstroms, or about 500-1500 angstroms so thefront-side semiconductor layer 110 has the same crystalline direction asthe back-side semiconductor layer 114.

The back-side semiconductor layer 114, the absorption enhancementsemiconductor layer 112, and the front-side semiconductor layer 110define a composite substrate 106. The front-side semiconductor layer 110is along and defines a front-side surface 106 f of the compositesubstrate 106, and the back-side semiconductor layer 114 is along anddefines a back-side surface 106 b of the composite substrate 106. Theback-side surface 106 b and the front-side surface 106 f are on oppositesides of the composite substrate 106.

As illustrated by the cross-sectional view 1500 of FIG. 15, a pluralityof absorption enhanced pixel sensors 306 a-306 c are formed on thefront-side surface 106 f of the composite substrate 106. Each of theabsorption enhanced pixel sensors 306 a-306 c comprises a photodetector120 defined by the front-side semiconductor layer 110, the absorptionenhancement semiconductor layer 112, and the back-side semiconductorlayer 114. For ease of illustration, only some of the photodetectors 120are labeled 120.

Each photodetector 120 of the absorption enhanced pixel sensors 306a-306 c comprises a first semiconductor region 120 f, a secondsemiconductor region (not labeled), and a photo junction 120 p. For easeof illustration, only some of the first semiconductor regions 120 f arelabeled 120 f, and only some of the photo junctions 120 pare labeled 120p. In some embodiments, each first semiconductor region 120 f of theabsorption enhanced pixel sensors 306 a-306 c is defined wholly orpartially by a front-side CCR 110 c of the front-side semiconductorlayer 110, an absorption enhancement CCR 112 c of the absorptionenhancement semiconductor layer 112, a first back-side CCR 114 c′ of theback-side semiconductor layer 114, and a second back-side CCR 114 cofthe back-side semiconductor layer 114. For ease of illustration, onlysome of the front-side CCRs 110 c are labeled 110 c, only some of theabsorption enhancement CCRs 112 c are labeled 112 c, only some of thefirst back-side CCRs 114 c′ are labeled 114 c, and only some of thesecond back-side CCRs 114 care labeled 114 c. In some embodiments, thesecond semiconductor regions of the of the absorption enhanced pixelsensors 306 a-306 c are defined wholly or partially by a front-side bulkregion 110 b of the front-side semiconductor layer 110, an absorptionenhancement bulk region 112 b of the absorption enhancementsemiconductor layer 112, a first back-side bulk region 114 b′ of theback-side semiconductor layer 114, and a second back-side bulk region114 b of the back-side semiconductor layer 114. For ease ofillustration, only some segments of the front-side bulk region 110 b arelabeled 110 b, and only some segments of the absorption enhancement bulkregion 112 b are labeled 112 b, and only some segments of the secondback-side bulk region 114 b are labeled 114 b.

In some embodiments, a process for forming the photodetectors 120comprises implanting dopants into the composite substrate 106 to formthe first semiconductor regions 120 f. In some of such embodiments, thecomposite substrate 106 initially has a single doping type, such thatthe doping is performed using dopants of an opposite doping type. Thedoping may, for example, be performed using ion implantation or someother doping process, and/or may, for example, use a photoresist mask toselectively implant the dopants.

Each of the absorption enhanced pixel sensors 306 a-306 c furthercomprises a transfer transistor 202. For ease of illustration, only someof the transfer transistors 202 are labeled 202. Each transfertransistor 202 of the absorption enhanced pixel sensors 306 a-306 ccomprises a gate electrode 204, a gate dielectric layer 206, a firstsource/drain region (not labeled), and a second source/drain region (notlabeled). For ease of illustration, only some of the gate electrodes 204are labeled 204 and only some of the gate dielectric layers 206 arelabeled 206. The first source/drain region may be defined by, forexample, a respective one of the first semiconductor regions 120 f ofthe photodetectors 120. The second source/drain region may be definedby, for example, a FDR 110 f of the front-side semiconductor layer 110.The transfer transistors 202 may, for example, be as described withregard to FIG. 10. Further, the transfer transistors 202 may, forexample, be formed as described with regard to FIG. 10.

Also illustrated by the cross-sectional view 1000 of FIG. 10, anisolation structure 208 is formed extending into the front-sidesemiconductor layer 110 to separate and surround the absorption enhancedpixel sensors 306 a-306 c. The isolation structure 208 may, for example,be as described with regard to FIG. 10 and/or formed as described withregard to FIG. 10.

As illustrated by the cross-sectional view 1600 of FIG. 16, aninterconnect structure 210 is formed covering the the absorptionenhanced pixel sensors 306 a-306 c and the composite substrate 106. Theinterconnect structure 210 comprises an ILD layer 212, a plurality ofwires 214, and a plurality of vias 216. For ease of illustration, onlysome of the wires 214 are labeled 214, and only some of the vias 216 arelabeled 216. The interconnect structure 210 may, for example, be asdescribed with regard to FIG. 11. Further, the interconnect structure210 may, for example, be formed as described with regard to FIG. 11.

As illustrated by the cross-sectional view 1700 of FIG. 17, in someembodiments, the front-side surface 106 f of the composite substrate 106is bonded to a carrier substrate 402 through the ILD layer 212. Thecarrier substrate 402 may be, for example, a bulk monocrystallinesilicon substrate, some other silicon substrate, a SOI substrate, asemiconductor substrate, or some other substrate. In some embodiments,the bonding process is performed by a fusion bonding process or someother bonding process.

As illustrated by the cross-sectional view 1800 of FIG. 18, thestructure of FIG. 17 is flipped vertically. Further, the compositesubstrate 106 is thinned at the back-side surface 106 b of the compositesubstrate 106, thereby reducing a thickness of the composite substrate106 and a thickness of the back-side semiconductor layer 114. In someembodiments, the thinning also removes the first back-side semiconductorsublayer 114 f. The thinning reduces a thickness of the compositesubstrate 106 from a first thickness T₁ to a second thickness T₂. Insome embodiments, the composite substrate 106 is thinned by performingan etch and/or a planarization into the back-side semiconductor layer114. The planarization may, for example, be performed by a CMP or someother planarization process.

In some embodiments, the first and second back-side semiconductorsublayers 114 f, 114 s are the same material and have the same dopingtype. Further, in some of such embodiments, the first and secondback-side semiconductor sublayers 114 f, 114 s have different dopingconcentrations. The different doping concentrations define a junction atan interface between the first and second back-side semiconductorsublayers 114 f, 114 s that may be used as a stop for the thinning,thereby allowing high degree of control over the thinning. For example,an etch may be performed as part of the thinning, where the etch uses anetchant that has a high selectivity for the first back-sidesemiconductor sublayer 114 f relative to the second back-sidesemiconductor sublayer 114 s, such that the junction serves as an etchstop. Such high selectivity may, for example, be between about 3:1-10:1,10:1-20:1, 20:1-50:1, or about 3:1-50:1.

As illustrated by the cross-sectional view 1900 of FIG. 19, a pluralityof protrusions 124 is formed in the composite substrate 106, includingthe back-side semiconductor layer 114. For ease of illustration, onlyone of the protrusions 124 is labeled 124. The protrusions 124 at leastpartially define an absorption enhancement structure 126. Theprotrusions 124 and the absorption enhancement structure 126 may, forexample, be as described with regard to FIG. 6. Further, a process forforming the protrusions 124 may, for example, be performed as describedwith regard to FIG. 6. In some embodiments, the process for forming theprotrusions 124 comprises performing an etch into the back-side surface106 b with a photoresist layer 602 in place, and subsequently removingthe photoresist layer 602.

In some embodiments, a systematic structure of pores is formed in theback-side semiconductor layer 114. The pores may be, for example,micrometer sized pores, nanometer sized pores, smaller sized pores, orany combination of the foregoing. In some embodiments, a process forforming the systematic structure of pores comprises performing an etchinto the back-side semiconductor layer 114. The process for forming thesystematic structure of pores may, for example, be performed before,during, or after the process for forming the protrusions 124.

As illustrated by cross-sectional view 2000 of FIG. 20, a passivationlayer 108 is formed covering the back-side surface 106 b of thecomposite substrate 106. The passivation layer 108 may, for example, beas described with regard to FIG. 7, and/or a process for forming thepassivation layer 108 may, for example, be as described with regard toFIG. 7.

As illustrated by the cross-sectional view 2100 of FIG. 21, a pluralityof color filters 404 and a plurality of micro-lenses 406 are formed onthe passivation layer 108. The color filters 404 respectively overliethe absorption enhanced pixel sensors 306 a-306 c, and the micro-lenses406 respectively overlie the color filters 404. The color filters 404may, for example, be as described with regard to FIG. 12, and/or may,for example, be formed as described with regard to FIG. 12. Themicro-lenses 406 may, for example, be as described with regard to FIG.12, and/or may, for example, be formed as described with regard to FIG.12.

With reference to FIG. 22, a flowchart 2200 of some embodiments of themethod of FIGS. 14-21 is provided.

At 2202, a back-side semiconductor layer is formed on a semiconductorsubstrate. See, for example, FIG. 14.

At 2202, an absorption enhancement semiconductor layer is formed on theback-side semiconductor layer. See, for example, FIG. 14.

At 2202, a front-side semiconductor layer is formed on the absorptionenhancement semiconductor layer. The front-side semiconductor layer hasa high energy bandgap relative to the absorption enhancementsemiconductor layer. The semiconductor substrate, the back-sidesemiconductor layer, the absorption enhancement semiconductor layer, andthe front-side semiconductor layer define a composite substrate. See,for example, FIG. 14.

At 2204, a pixel sensor is formed on a front-side surface of thecomposite substrate. The pixel sensor comprises a photodetectorpartially defined by the absorption enhancement semiconductor layer.See, for example, FIG. 15.

At 2206, an interconnect structure is formed covering the pixel sensoron the front-side surface of the composite substrate. The interconnectstructure comprises an ILD layer, wires, and vias. The wires and thevias are alternatingly stacked in the ILD layer. See, for example, FIG.16.

At 2208, a carrier substrate is bonded to a front-side surface of thecomposite substrate through the ILD layer. See, for example, FIG. 17.

At 2210, the composite substrate is thinned at a back-side surface ofthe composite substrate. The thinning removes the semiconductorsubstrate. See, for example, FIG. 18.

At 2212, a plurality of protrusions is formed along the back-sidesurface of the composite substrate. The protrusions at least partiallydefine an absorption enhancement structure. See, for example, FIG. 19.

At 2214, a passivation layer is formed on the back-side surface of thecomposite substrate. See, for example, FIG. 20.

At 2216, a color filter and a micro-lens are formed stacked on thepassivation layer. See, for example, FIG. 21.

While the flowchart 2200 of FIG. 22 is illustrated and described hereinas a series of acts or events, it will be appreciated that theillustrated ordering of such acts or events is not to be interpreted ina limiting sense. For example, some acts may occur in different ordersand/or concurrently with other acts or events apart from thoseillustrated and/or described herein. Further, not all illustrated actsmay be required to implement one or more aspects or embodiments of thedescription herein, and one or more of the acts depicted herein may becarried out in one or more separate acts and/or phases.

In some embodiments, the present application provides an image sensorincluding: a front-side semiconductor layer, an absorption enhancementsemiconductor layer, and a back-side semiconductor layer that arestacked, wherein the absorption enhancement semiconductor layer isstacked between the front-side and back-side semiconductor layers, andwherein the absorption enhancement semiconductor layer has an energybandgap less than that of the front-side semiconductor layer; aplurality of protrusions defined by the back-side semiconductor layer;and a photodetector defined by the front-side semiconductor layer, theabsorption enhancement semiconductor layer, and the back-sidesemiconductor layer. In some embodiments, the front-side semiconductorlayer includes silicon, wherein the absorption enhancement semiconductorlayer includes germanium or silicon germanium. In some embodiments, thefront-side semiconductor layer includes silicon, wherein the absorptionenhancement semiconductor layer includes silicon doped with a chalcogen.In some embodiments, the energy bandgap of the absorption enhancementsemiconductor layer is less than about 1 electron volt (eV), and theenergy bandgap of the front-side semiconductor layer is greater thanabout 1 eV. In some embodiments, the front-side semiconductor layerdirectly contacts the absorption enhancement semiconductor layer at afront-side heterojunction, wherein the back-side semiconductor layerdirectly contacts the absorption enhancement semiconductor layer at aback-side heterojunction. In some embodiments, the image sensor furtherincludes: a front-side buffer layer between the front-side semiconductorlayer and the absorption enhancement semiconductor layer; and aback-side buffer layer between the back-side semiconductor layer and theabsorption enhancement semiconductor layer; wherein the front-side andback-side buffer layers include semiconductor material with an elevatedconcentration of carbon relative to the front-side semiconductor layer,the absorption enhancement semiconductor layer, and the back-sidesemiconductor layer. In some embodiments, the plurality of protrusionshas a saw-toothed profile. In some embodiments, the back-sidesemiconductor layer includes nanoporous silicon, wherein the protrusionsand the nanoporous silicon define an absorption enhancement structurewith a direct energy bandgap. In some embodiments, the image sensorfurther includes: a transfer transistor on and partially defined by thefront-side semiconductor layer, wherein the transfer transistor bordersand is electrically coupled to the photodetector.

In some embodiments, the present application provides a method formanufacturing an image sensor, the method including: providing afront-side semiconductor layer, an absorption enhancement semiconductorlayer, and a back-side semiconductor layer that are stacked, wherein theabsorption enhancement semiconductor layer is stacked between thefront-side and back-side semiconductor layers, and wherein theabsorption enhancement semiconductor layer has an energy bandgap lessthan that of the front-side semiconductor layer; performing an etch intothe back-side semiconductor layer to form a plurality of protrusions;and forming a photodetector in the front-side semiconductor layer, theabsorption enhancement semiconductor layer, and the back-sidesemiconductor layer. In some embodiments, the etch is performed beforethe forming of the photodetector. In some embodiments, the methodfurther includes: thinning the front-side semiconductor layer to reducea thickness of the front-side semiconductor layer; and after thethinning, forming a pixel sensor on the front-side semiconductor layer,wherein the pixel sensor includes the photodetector and a transfertransistor, and wherein the transfer transistor is defined in part bythe front-side semiconductor layer. In some embodiments, the etch isperformed after the forming of the photodetector. In some embodiments,the method further includes: forming a pixel sensor on the front-sidesemiconductor layer, wherein the pixel sensor includes the photodetectorand a transfer transistor, and wherein the transfer transistor isdefined in part by the front-side semiconductor layer; and after theforming of the pixel sensor, thinning the back-side semiconductor layerto reduce a thickness of the back-side semiconductor layer, wherein theetch is performed after the thinning. In some embodiments, the back-sidesemiconductor layer includes a first back-side semiconductor sublayerand a second back-side semiconductor sublayer, wherein the secondback-side semiconductor sublayer is stacked between the first back-sidesemiconductor sublayer and the absorption enhancement semiconductorlayer, and wherein the thinning removes the first back-sidesemiconductor sublayer. In some embodiments, the first and secondback-side semiconductor sublayers have the same material and the samedoping type, wherein the first and second back-side semiconductor layershave different doping concentrations, and wherein the thinning includesetching the first back-side semiconductor sublayer until the firstback-side semiconductor sublayer is removed. In some embodiments, thefront-side semiconductor layer includes silicon, wherein the absorptionenhancement semiconductor layer includes: 1) germanium; 2) silicongermanium; or 3) crystalline silicon doped with a chalcogen. In someembodiments, the energy bandgap of the absorption enhancementsemiconductor layer is less than about 1electron volt (eV), wherein theenergy bandgap of the front-side semiconductor layer is greater thanabout 1 eV. In some embodiments, the protrusions are formed in aperiodic pattern.

In some embodiments, the present application provides another imagesensor including: a composite substrate including a front-sidesemiconductor layer, an absorption enhancement semiconductor layer, anda back-side semiconductor layer that are stacked, wherein the front-sidesemiconductor layer defines a front-side surface of the compositesubstrate, wherein the back-side semiconductor layer defines a back-sidesurface of the composite substrate that is opposite the front-sidesurface of the composite substrate, wherein the absorption enhancementsemiconductor layer is stacked between the front-side and back-sidesemiconductor layers, and wherein the absorption enhancementsemiconductor layer has an energy bandgap less than that of thefront-side semiconductor layer; a periodic structure of protrusionsdefined by the back-side semiconductor layer and along the back-sidesurface of the composite substrate; a passivation layer directly on theback-side surface of the composite substrate, wherein the passivationlayer conforms to the protrusions; and a pixel sensor directly on thefront-side surface of the composite substrate, wherein the pixel sensorincludes a photodetector defined by the front-side semiconductor layer,the absorption enhancement semiconductor layer, and the back-sidesemiconductor layer, wherein the pixel sensor includes a transfertransistor defined in part by the front-side semiconductor layer, andwherein the transfer transistor borders and is electrically coupled tothe photodetector.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An image sensor comprising: a front-sidesemiconductor layer, an absorption enhancement semiconductor layer, anda back-side semiconductor layer that are stacked, wherein the absorptionenhancement semiconductor layer is stacked between the front-side andback-side semiconductor layers, and wherein the absorption enhancementsemiconductor layer has an energy bandgap less than that of thefront-side semiconductor layer; a plurality of protrusions defined bythe back-side semiconductor layer; and a photodetector defined by thefront-side semiconductor layer, the absorption enhancement semiconductorlayer, and the back-side semiconductor layer.
 2. The image sensoraccording to claim 1, wherein the front-side semiconductor layercomprises silicon, and wherein the absorption enhancement semiconductorlayer comprises germanium or silicon germanium.
 3. The image sensoraccording to claim 1, wherein the front-side semiconductor layercomprises silicon, and wherein the absorption enhancement semiconductorlayer comprises silicon doped with a chalcogen.
 4. The image sensoraccording to claim 1, wherein the energy bandgap of the absorptionenhancement semiconductor layer is less than about 1 electron volt (eV),and wherein the energy bandgap of the front-side semiconductor layer isgreater than about 1 eV.
 5. The image sensor according to claim 1,wherein the front-side semiconductor layer directly contacts theabsorption enhancement semiconductor layer at a front-sideheterojunction, and wherein the back-side semiconductor layer directlycontacts the absorption enhancement semiconductor layer at a back-sideheterojunction.
 6. The image sensor according to claim 1, furthercomprising: a front-side buffer layer between the front-sidesemiconductor layer and the absorption enhancement semiconductor layer;and a back-side buffer layer between the back-side semiconductor layerand the absorption enhancement semiconductor layer; wherein thefront-side and back-side buffer layers comprise semiconductor materialwith an elevated concentration of carbon relative to the front-sidesemiconductor layer, the absorption enhancement semiconductor layer, andthe back-side semiconductor layer.
 7. The image sensor according toclaim 1, wherein the plurality of protrusions has a saw-toothed profile.8. The image sensor according to claim 1, wherein the back-sidesemiconductor layer comprises nanoporous silicon, and wherein theprotrusions and the nanoporous silicon define an absorption enhancementstructure with a direct energy bandgap.
 9. The image sensor according toclaim 1, further comprising: a transfer transistor on and partiallydefined by the front-side semiconductor layer, wherein the transfertransistor borders and is electrically coupled to the photodetector. 10.A method for manufacturing an image sensor, the method comprising:providing a front-side semiconductor layer, an absorption enhancementsemiconductor layer, and a back-side semiconductor layer that arestacked, wherein the absorption enhancement semiconductor layer isstacked between the front-side and back-side semiconductor layers, andwherein the absorption enhancement semiconductor layer has an energybandgap less than that of the front-side semiconductor layer; performingan etch into the back-side semiconductor layer to form a plurality ofprotrusions; and forming a photodetector in the front-side semiconductorlayer, the absorption enhancement semiconductor layer, and the back-sidesemiconductor layer.
 11. The method according to claim 10, wherein theetch is performed before the forming of the photodetector.
 12. Themethod according to claim 10, further comprising: thinning thefront-side semiconductor layer to reduce a thickness of the front-sidesemiconductor layer; and after the thinning, forming a pixel sensor onthe front-side semiconductor layer, wherein the pixel sensor comprisesthe photodetector and a transfer transistor, and wherein the transfertransistor is defined in part by the front-side semiconductor layer. 13.The method according to claim 10, wherein the etch is performed afterthe forming of the photodetector.
 14. The method according to claim 10,further comprising: forming a pixel sensor on the front-sidesemiconductor layer, wherein the pixel sensor comprises thephotodetector and a transfer transistor, and wherein the transfertransistor is defined in part by the front-side semiconductor layer; andafter the forming of the pixel sensor, thinning the back-sidesemiconductor layer to reduce a thickness of the back-side semiconductorlayer, wherein the etch is performed after the thinning.
 15. The methodaccording to claim 14, wherein the back-side semiconductor layercomprises a first back-side semiconductor sublayer and a secondback-side semiconductor sublayer, wherein the second back-sidesemiconductor sublayer is stacked between the first back-sidesemiconductor sublayer and the absorption enhancement semiconductorlayer, and wherein the thinning removes the first back-sidesemiconductor sublayer.
 16. The method according to claim 15, whereinthe first and second back-side semiconductor sublayers have the samematerial and the same doping type, wherein the first and secondback-side semiconductor layers have different doping concentrations, andwherein the thinning comprises etching the first back-side semiconductorsublayer until the first back-side semiconductor sublayer is removed.17. The method according to claim 10, wherein the front-sidesemiconductor layer comprises silicon, and wherein the absorptionenhancement semiconductor layer comprises: 1) germanium; 2) silicongermanium; or 3) crystalline silicon doped with a chalcogen.
 18. Themethod according to claim 10, wherein the energy bandgap of theabsorption enhancement semiconductor layer is less than about 1 electronvolt (eV), and wherein the energy bandgap of the front-sidesemiconductor layer is greater than about 1 eV.
 19. The method accordingto claim 10, wherein the protrusions are formed in a periodic pattern.20. An image sensor comprising: a composite substrate comprising afront-side semiconductor layer, an absorption enhancement semiconductorlayer, and a back-side semiconductor layer that are stacked, wherein thefront-side semiconductor layer defines a front-side surface of thecomposite substrate, wherein the back-side semiconductor layer defines aback-side surface of the composite substrate that is opposite thefront-side surface of the composite substrate, wherein the absorptionenhancement semiconductor layer is stacked between the front-side andback-side semiconductor layers, and wherein the absorption enhancementsemiconductor layer has an energy bandgap less than that of thefront-side semiconductor layer; a periodic structure of protrusionsdefined by the back-side semiconductor layer and along the back-sidesurface of the composite substrate; a passivation layer directly on theback-side surface of the composite substrate, wherein the passivationlayer conforms to the protrusions; and a pixel sensor directly on thefront-side surface of the composite substrate, wherein the pixel sensorcomprises a photodetector defined by the front-side semiconductor layer,the absorption enhancement semiconductor layer, and the back-sidesemiconductor layer, wherein the pixel sensor comprises a transfertransistor defined in part by the front-side semiconductor layer, andwherein the transfer transistor borders and is electrically coupled tothe photodetector.